More volume of fluid will not keep it cooler just take longer to get there. I live in Phoenix area during the summer we start at 120F before we even turn the key. Transmission fluid has a constant expansion rate, when you add more fluid it will expand more. It appears transmission fluid has roughly an expansion coefficient of .07% per degree C. Allison holds 12.7 quarts, so if you start at 100F would be 13.589 quarts at 200F, an increase of .889. Add 4 quarts (PPE pan) you have 16.7 quarts at 100F would then be 17.869 quarts, an increase of 1.169. I agree with Mike you have to take into account the extra fluid by starting at a slightly lower starting point, how much I would listen to Mike he has probably been there done that to some extent. If you don’t trust him heat your transmission to 225F and set the level to top of the hot line.